This invention relates to synthetic acoustic impedance logs and more particularly, to obtaining synthetic acoustic impedance logs having improved characteristics below the seismic pass band.
Acoustic logs are widely used in geophysical exploration. Well logging tools such as the one shown in U.S. Pat. No. 3,312,934--Stripling et al traverse a well bore to obtain logs of the acoustic velocity characteristics of the earth as a function of depth. Alternatively, the logs represent the acoustic impedance which is the product of acoustic velocity and density. As used herein, the term acoustic log means either acoustic velocity or acoustic impedance.
When there is not a well bore in the vicinity of the area being explored, synthetic acoustic logs have been obtained from seismograms which are shot from the surface of the earth. U.S. Pat. Nos. 3,076,176--Lawrence and 3,076,177--Lawrence et al describe the production of synthetic velocity logs from seismograms.
More recently, velocity spectra techniques have been used to obtain acoustic velocity from seismograms. U.S. Pat. No. 3,611,278--Guinzy and Ruehle, is an example of a velocity spectra technique which can be used to obtain an interval velocity log from seismograms.
Recent advances in the processing of seismograms have greatly improved the signal-to-noise ratio of the seismograms. In particular, common depth point shooting and processing techniques have greatly improved the appearance of reflections from common reflection points. In this procedure the field seismograms which contain reflections from common points are composited, or stacked, so that the reflection signals are emphasized relative to the noise.
Synthetic acoustic logs have been obtained from common depth point seismograms. However, these acoustic logs do not have good characteristics at frequencies below the seismic pass band.
It is an object of the present invention to produce synthetic acoustic logs which do have good characteristics at frequencies below the seismic band.